ASE A8 fuel, air induction and exhaust systems practice questions.

Five free practice questions on fuel, air induction and exhaust systems: content area C of the ASE A8 Engine Performance test, 9 of its 50 scored questions (18%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Recall · Fuel, Air Induction and Exhaust Systems Diagnosis and Repair

   At warm idle the scan tool shows short-term fuel trim at +2% and long-term fuel trim at +20%. What does this tell the technician?

   1. The PCM is taking out fuel to correct a rich condition
   2. The mixture is 22% leaner than its target right now
   3. The PCM has learned to add fuel for a lean condition
   4. The oxygen sensors are stuck reporting a rich mixture
   Show the answer

   C. Correct. Long-term trim is the correction the PCM has learned and stored over time; +20% means it is adding about a fifth more fuel than its base calculation to correct a lean condition. Short-term trim near zero shows that with the learned correction in place the mixture is now on target, so the cause of the lean condition still has to be found.

   Why not the others

   A. Positive trim means fuel is being added. A rich condition would make the PCM take fuel out, and both numbers would read negative.

   B. Fuel trim is the correction being applied, not the error left over. With the learned +20% in place, short-term trim near zero shows the mixture is now on target; the 22% total (2 + 20) is how much extra fuel it takes to get there.

   D. A sensor stuck reporting rich would make the PCM remove fuel, driving trims negative. These positive trims show the sensors have been reporting lean until the added fuel brought the mixture back.

2. Recall · Fuel, Air Induction and Exhaust Systems Diagnosis and Repair

   Why is a metal fuel filler neck connected to the body with a ground strap?

   1. To keep the fuel tank pressure sensor reading steady
   2. To give the fuel level sender its ground path
   3. To drain static charge built up during refueling
   4. To stop galvanic corrosion between the neck and body
   Show the answer

   C. Correct. Fuel flowing through the neck can build a static charge, and a metal neck isolated by rubber hoses could hold that charge until it sparks at the nozzle. The strap bonds the neck to the body so the charge drains away; a missing or broken strap is a repair item, not a cosmetic one.

   Why not the others

   A. The tank pressure sensor has its own reference and ground wires to the PCM; a strap on the filler neck plays no part in its signal. The strap is there so the neck cannot hold a static charge.

   B. The sender is grounded through its own circuit at the pump module and is not wired through the filler neck. The neck's strap bonds it to the body so static cannot build up on it.

   D. Bonding two metals together electrically does nothing to prevent galvanic corrosion; coatings and seals do that job. The strap keeps the neck at body potential so flowing fuel cannot build a static charge on it.

3. Recall · Fuel, Air Induction and Exhaust Systems Diagnosis and Repair

   A fuel sample drawn from the tank of a vehicle running on gasoline with 10% ethanol settles in a clear container into two layers, with a cloudy layer at the bottom. What is the bottom layer?

   1. Water and ethanol separated out of the fuel
   2. Diesel fuel that was mixed into the gasoline
   3. Fuel system cleaner that has settled out
   4. Gum left by gasoline that has gone stale
   Show the answer

   A. Correct. Gasoline with ethanol holds a little water in solution, but once it takes on more than it can hold, the water and much of the ethanol drop out together as a separate layer at the bottom. The pump can pick that layer up and stall the engine, so the tank and lines are drained and refilled with fresh fuel.

   Why not the others

   B. Diesel blends into gasoline without separating, so a mix-up shows as odd color and odor and poor running, not as a distinct bottom layer. A cloudy lower layer in ethanol-blended fuel is water and ethanol that have separated out.

   C. Fuel system cleaners are made to dissolve in gasoline and do not settle into a separate layer. A cloudy layer at the bottom of an ethanol blend is water that has pulled ethanol out of the fuel.

   D. Stale gasoline darkens and leaves sticky varnish and gum on parts, but it does not form a cloudy liquid layer in a sample. That layer is water and ethanol that have separated from the fuel.

4. Recall · Fuel, Air Induction and Exhaust Systems Diagnosis and Repair

   An intake manifold has variable-length runners controlled by the PCM. At low engine speed, which runner path is selected, and why?

   1. The short path, to lower intake air noise
   2. The short path, to boost low-speed torque
   3. The long path, to raise power at high speed
   4. The long path, to boost low-speed torque
   Show the answer

   D. Correct. A long runner holds a longer column of air whose pressure waves arrive at the intake valve in step with the slower valve events, packing more air into the cylinder and building torque. At higher speed the PCM switches to the short path, which is tuned for the faster valve events and top-end power.

   Why not the others

   A. Runner length is switched for engine breathing, not noise, and the short path is the high-speed setting. At low speed the long path builds torque.

   B. Short runners are tuned to high engine speed, where the valve events come faster; at low speed they give up torque. The long path is used at low speed.

   C. The long path is used at low speed, but its purpose is torque there, not high-speed power; the system switches to the short path for high-speed power.

5. Diagnosis · Fuel, Air Induction and Exhaust Systems Diagnosis and Repair

   A V6 MAF engine has long-term fuel trim of +16% on bank 1 and +1% on bank 2 at idle, and +8% and +1% at cruise. A smoke test finds no intake leaks, and fuel pressure is within specification. Which of these could be the cause?

   1. An air leak at a bank 1 intake runner
   2. An injector on bank 1 leaking past its valve
   3. A fuel filter restricting flow to the rail
   4. An exhaust leak ahead of the bank 1 sensor
   Show the answer

   D. Correct. Exhaust pulses through a leak draw in outside air between them, so the upstream sensor sees extra oxygen and reports lean; the PCM adds fuel to bank 1 even though the mixture was correct. The effect is largest at idle, when exhaust flow is lowest, and smaller at cruise.

   Why not the others

   A. An intake runner leak could lean bank 1 alone, but the smoke test found no intake leaks. With the intake sealed and fuel pressure normal, the extra oxygen is getting into bank 1's exhaust ahead of the sensor.

   B. A leaking injector adds fuel on bank 1, which drives that bank's trim negative. Bank 1's trim is positive, so the PCM is adding fuel to correct what its sensor reports as lean.

   C. Both banks are fed through the same filter, so a restriction would lean both banks, and most at high demand. Only bank 1 is high, and it is worst at idle.

02Answer key

03What area C covers

This content area covers how fuel and air reach the cylinders and how exhaust leaves: tanks, lines, pumps and filters, pump control circuits, pressure control in return, returnless and direct injection systems, fuel quality, injectors, throttle bodies, air filters and intake manifolds, vacuum and unmetered air leaks, exhaust leaks and restrictions, oxygen sensors, and turbochargers and superchargers.

The ASE test specification calls this content area “Fuel, Air Induction and Exhaust Systems Diagnosis and Repair”. It carries 9 of the 50 scored questions, 18% of the test.

• Diagnose fuel-related starting, idle and driveability faults
• Analyze fuel trim and fuel and air DTCs
• Test fuel pumps, control circuits and pressure regulation
• Check fuel for water and alcohol content
• Service throttle bodies, injectors and fuel rails
• Find vacuum leaks, restrictions and unmetered air
• Test exhaust systems for leaks and restriction
• Inspect turbochargers, superchargers and charge-air parts

04What the questions turn on

Short-term and long-term trim

Long-term fuel trim is the correction the PCM has learned and stored over time. A high positive long-term trim with short-term trim near zero means the learned correction is holding the mixture on target, and the cause of the lean condition is still there.

Pressure is not volume

A pump can hold specified pressure at idle flow and still fall short on volume under full-load demand. When the volume test fails with good voltage at the pump, the filter, strainer and lines are checked for a restriction.

Air the MAF never sees

Only air that enters after the MAF sensor is unmetered. A torn air filter element sits ahead of the sensor, so its air is still measured. A split hose after a turbo loses air that was measured, so the mixture runs rich under boost.

Exhaust leaks fool oxygen sensors

An exhaust leak ahead of an upstream sensor draws in outside air between pulses, so the sensor reports lean and the PCM adds fuel to that bank even though the mixture was correct. The effect is largest at idle, when exhaust flow is lowest.

05The other content areas

The A8 test has five content areas. Each has its own page of free questions:

The ASE A8 Engine Performance practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.

ToolboxPrep is an independent study site. It is not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original practice questions written to the published ASE test specifications; they are not actual test questions.